Question: In chemistry class, the girl to boy ratio is $9$ to $10$. If there are a total of $38$ students, how many girls are there?
Explanation: A ratio of $9$ girls to $10$ boys means that a set of $19$ students will have $9$ girls and $10$ boys. A class of $38$ students has $2$ sets of $19$ students. Because we know that there are $9$ girls in each set of $19$ students, the class must have $2$ groups of $9$ girls each. There is a total of $18$ girls in chemistry class.